1. Field of the Invention
The present invention relates to a light-sensitive lithographic printing plate (or so-called presensitized plate) and more particularly to an improved light-sensitive lithographic printing plate in which a light-sensitive layer has a particular constitution.
2. Description of the Prior Art
The light-sensitive materials previously used in presensitized printing plates are, for the most part, diazonium compounds and of such compounds, the most frequently used are diazo resins such as the formaldehyde condensates of p-diazodiphenylamines. On the occasion that such a diazo resin is coated on an appropriate support such as a sheet of paper, a plastic film, a metal plate or the like and the resulting coating is exposed to actinic radiation through a negative transparency, the diazo resin present undergoes photodecomposition in the exposed areas and is converted into water-insoluble substances (such portions are hereinafter referred to as the image areas). On the other hand, the unexposed areas are subsequently washed out with water and thereby the surface of the support is exposed in areas corresponding to the unexposed areas (such portions are hereinafter referred to as non-image areas). When the support used has been subjected to a preliminary treatment so as to render the surface thereof hydrophilic, the hydrophilic surface is exposed in the unexposed areas through development. Accordingly, this area accepts water and repels greasy ink on an offset printing machine. On the other hand, the photodecomposition products of a diazo resin display an oleophilic property and repel water and accept greasy ink thereon. Light-sensitive printing plates of this kind are the so-called negative working light-sensitive printing plates.
Compositions of the light-sensitive layers suitable for the light-sensitive printing plates of the above-described kind can be grouped into two classes: One class includes compositions consisting of diazo resins alone, namely compositions free from binders, as described, for example, in U.S. Pat. No. 2,714,066; and the other class includes compositions consisting of combinations of diazo resins and binders, as described, for example, in U.S. Pat. No. 2,826,501 and British Pat. No. 1,074,392. In the former case, the thickness of the light-sensitive layer is on the order of 20 to 100 mg/m.sup.2 (converted to a basis of coated weight) due to the absence of a binder. The light-sensitive layer can only provide an image possessing low abrasion resistance and short press life as it stands. Therefore, it is necessary to strengthen the image areas. In order to afford printing plates that can resist various kinds of mechanical forces such as friction, peeling and like forces, which occur during the printing process certain resins are selectively fixed on the image areas using lacquers in the form of emulsions simultaneously with or after the development-processing. Lacquers employed for such purpose must have excellent abrasion resistance, be able to adhere to the image part and not likely to cause catch up, scum and the like. U.S. Pat. No. 2,754,279 discloses an improved lacquer composition. However, since plate-making operators coat lacquers on the lithographic printing plates and further many of the abilities of the printing plates obtained depend on whether lacquers are coated thereon skillfully or unskillfully, great skill is required. With the intention of overcoming such disadvantages, light-sensitive lithographic printing plates incorporating in the light-sensitive layers together with diazo resins, resins excellent in abrasion resistance corresponding to the above-described lacquers, and in which the light-sensitive layers are adjusted so as to have the requisite thickness (about 0.5 to 5 g/m.sup.2) have been proposed as disclosed in each of U.S. Pat. No. 2,826,501 and British Pat. No. 1,074,392. When the light-sensitive lithographic printing plate of this kind is exposed to light through an image-bearing transparency and subsequently development-processed using a certain developing solution, the unexposed areas of the light-sensitive layer can be removed from the base and the exposed areas of the light-sensitive layer are left on the base, because the exposed areas possess lower solubility in the developing solution or lower facility upon swelling in the developing solution than the unexposed areas do. Accordingly, the thus-obtained lithographic printing plates exhibit excellent printabilities equivalent or superior to those attained in the case that the above-described lacquers are coated upon the image areas after development.
However, in the case of the incorporation of resins into the light-sensitive layers the compatibilities of the resins used with the diazo resins and the selection of common solvents are important, and the resins contained as binders in the light-sensitive layers must have such a property that the exposed areas of the light-sensitive layer containing the resin functioning as a binder becomes, on the whole, insoluble in the developing solution through photodecomposition of the light-sensitive substance, or there is a clear difference in solubility or in the degree of swelling between the exposed and the unexposed areas, namely excellent hardening property. Further the coexistence of the resin with the light-sensitive diazo resin cannot cause deterioration in the storage stability of the light-sensitive layer. That is, properties of the light-sensitive layer such as sensitivity, developability, etc., are maintained at fresh levels upon storage. It has been known that certain water-soluble colloids can produce resists of very high quality when employed as binders for diazo resins but they deteriorate in a few days due to a dark reaction and lose their sensitivities to light.
As resins possessing many of the properties required for the above-described purposes to a satisfactory extent, shellac as described in British Pat. No. 1,350,521; homopolymers of hydroxyalkylacrylates or hydroxyalkylmethacrylates or copolymers prepared from either of the above-described monomers and one or more of a copolymerizable monomer, as described in British Pat. Nos. 1,460,978; 1,475,599 and 1,505,739; and so on have been known.
However, even in the lithographic printing plates made using light-sensitive lithographic printing plates having light-sensitive layers comprising such binders and diazo resins, the sensitivity to grease (namely the ability to accept greasy ink employed for lithographic printing) in the image areas is low. Therefore, when prints are produced in large numbers repeatedly using the same printing plate, the first sheets of prints have low image densities and only upon subsequent printing are prints of desired image densities obtained. Accordingly, the first sheets of prints having low image densities cannot but be discarded as spoilages. The solution of this problem has been sought earnestly by one skilled in the printing art for a long time.
Now, as a technical approach for producing light-sensitive lithographic printing plates which can afford lithographic printing plates having image areas possessing high oleophilic property (i.e., affinity to grease), a method in which a small amount of binder having a strong oleophilic character is added to the light-sensitive layer has been adopted. For instance, U.S. Pat. No. 4,123,297 corresponding to Japanese Patent Application (OPI) No. 125806/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a process for imparting a high affinity to grease to the light-sensitive layer in which a strongly oleophilic novolak type t-butylphenol-formaldehyde resin is homogeneously incorporated in small amounts into the light-sensitive layer comprising an o-quinonediazide and a novolak type cresol-formaldehyde resin. However, there are various restrictions on the material such as the strongly oleophilic binder employed to increase the oleophilic property must possess good compatibility with the binder employed for improving the press life and must not reduce developability. Under these circumstances, such an approach can be applied only with an extremely limited range of binders. In practice, although the above-described approach has been applied to the negative type light-sensitive lithographic printing plates disclosed in British Pat. No. 1,475,599 for the purpose of producing lithographic printing plates more excellent in the affinity to grease, satisfactory results could not be obtained.